This book chapter presents three methodological challenges stemming from archival research. The chapter reflects upon these challenges and provides solutions based upon personal experiences with legal historical analysis into the ‘fair and equitable treatment’ (FET) clause, found today in most international investment agreements. The chapter proceeds in four parts. Part one discusses the first challenge, which refers to whether historical research can or should be divided into explicit time periods, also known as periodization. Part two examines the second challenge: defining the subject matter of the historical research, and deciding whether international legal history should exclusively focus on law by studying the history of rules and concepts, or whether to look at law in context, studying its practice, and at how legal institutions work. Part three examines the issue of anachronism in legal historical methodology, meaning whether to read the past with knowledge of the present. Part four concludes.